1. Field of the Invention
The present invention is related a communication device used in a wireless communication system, and more particularly, to a communication device of handling a clear channel assessment and a signal transmission in an unlicensed band in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system developed by the 3rd Generation Partnership Project (3GPP) is a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of data transmission. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
Network operators propose to offload network traffic of the LTE/LTE-A system to an unlicensed band, to ease load of the network traffic.
For transmission in the unlicensed band, listen before talk (LBT) may be needed. To meet the LBT requirement, a clear channel assessment (CCA) operation (e.g., CCA check) may be performed within a designated time period before a transmission is performed. In detail, the eNB may suspend a transmission on a downlink (DL) carrier in an unlicensed band for a first period to perform a first CCA operation on the DL carrier in the unlicensed band. In addition, the eNB may suspend another transmission for a second period and perform a second CCA operation for the second period, when the result of the first CCA operation indicates the DL carrier is busy. In this situation, the UE receives no data during the first period and the second period. Thus, throughput of the UE is decreased due to the suspensions of the transmissions related to the CCA operation. In addition, the suspension of the transmission during the first period and the second period may result in incorrect measurement results.
Hence, how to handle a CCA and the transmissions on the carrier (i.e., unlicensed carrier) in the unlicensed band is an important topic to be discussed.